Context: Vitamin D status can influence insulin resistance.

Objective: To determine the prevalence of vitamin D deficiency in obese African American (AA) adolescent females in a southeastern latitude, and to determine the relationship of 25-hydroxyvitamin D (25(OH)D) with insulin and glucose dynamics.

Designs: Cross sectional study in a University Children's hospital.

Methods: Serum 25(OH)D, fasting glucose, parathyroid hormone, serum calcium, serum lipids, serum transaminases, and C-reactive protein were assessed. Indices of insulin sensitivity and resistance were determined from an oral glucose tolerance test. Subjects were classified as vitamin D deficient or sufficient, based on the traditional vitamin D deficiency definition (serum 25(OH)D < 20 ng/ml) and also by a lower cut-point of 25(OH)D 15 ng/mL.

Results: 51 AA adolescent females (BMI 43.3 ± 9.9 kg/m² and age 14 ± 2 yr) were studied. Serum 25(OH)D concentrations were 20 ng/ml in 78.4% and 15 ng/ml in 60.8% of subjects. There were no significant group differences in the metabolic outcomes when subjects were classified using the traditional vitamin D deficiency definition. The Matsuda index of insulin sensitivity was significantly lower (P = 0.02) and insulin area under the curve was significantly higher (P = 0.04) in subjects with 25(OH)D concentrations 15 ng/mL vs. those with higher concentrations.

Conclusions: Vitamin D deficiency is highly prevalent in obese, AA female adolescents and may promote insulin resistance. Our data suggests that a 25(OH)D concentrations 15 ng/mL may be the threshold by which vitamin D deficiency confers negative effects on insulin sensitivity.